Planar shaped solar concentrators have advantages of light weight, compact volume and good compatibility with buildings. However, due to the limitation of shape and material, planar shaped concentrators usually have lower concentration ratio and lower concentration efficiency compared with other types of concentrators such as parabolic concentrator and concentrating lens, etc.
One kind of planar concentrators is luminescent solar concentrator that is typically made of transparent media, such as glass or plastic, containing luminescent material which absorbs solar radiation and emits electromagnetic radiation at a longer wavelength with an isotropic distribution of propagating direction. Most emitted radiation will be trapped in the transparent media by the total inner reflection of the top and bottom surfaces and can only escape through the edges of the planar concentrator. As discussed in U.S. Pat. No. 4,488,047, the efficiency of this kind of concentrators is usually not high due to multiple factors such as, radiation reabsorbed by the luminescor, transmission losses of the transparent media, and non-radiative losses by photo thermal conversion and photo chemical degradation.
Another type of planar concentrators is holographic planar concentrator which applies holographic elements to change direction of incident solar radiation and trap the energy by total inner reflection of the top and bottom surfaces of a transparent media. Earlier arts including U.S. Pat. No. 4,863,224, issued to Afian et al. and U.S. Pat. No. 5,268,985 issued to Ando et al. disclose holographic concentrators and holographic light guides with single angle of incidence. U.S. Pat. Nos. 5,877,874 and 6,274,860 both issued to Rosenberg disclose a holographic planar concentrator using angularly and spectrally multiplexed holographic film to enable passive solar tracking ability and using spatial multiplexing to improve optical recoupling loss.
At current stage, holographic planar concentrators (HPC) can generally achieve higher efficiency than luminescent solar concentrators without the needs for active tracking. However, the length of HPC in light guiding direction is difficult to be increased due to optical recoupling loss, therefore the concentration ratio is low. Increasing concentration ratio will either need thicker plane of transparent material which result in higher cost and weight or need spatial multiplexing which increase cost and complexity to make. Other than that, the transparent material will also cause energy loss for light to travel over long distance. Higher transparency material will further increase cost.
Using an angle selective optical filter can be another approach to collect radiant energy. There are a few patents related to the angle selective optical filter. U.S. Pat. No. 6,992,830 issued to Mitchell et al. discloses a projection display having an angle selective coating for enhanced image contrast. U.S. Pat. No. 7,108,383 issued to Mitchell. discloses an optical system includes a reflection angle selective mirror. U.S. Pat. No. 6,667,794 issued to Arakawa et al. discloses a collimator using wavelength selective reflection system which comprises either cholesteric liquid crystal layers or a dielectric multilayer film. However, using an angle selective filter for concentrating and transferring radiant energy has not been proposed yet.